connections and bracing - presentation
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1Connections and Bracing
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2
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3Forces On Structures Forces from gravity, wind,
and seismic events areimposed on all structures
Forces that act vertically aregravity loads
Forces that act horizontally,such as stability, wind andseismic events (the focus ofthis discussion) requirelateral load resisting systemsto be built into structures
As lateral loads are appliedto a structure, horizontaldiaphragms (floors androofs) transfer the load to thelateral load resisting system
Structural Steel Frame Elevation
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4Initial System Planning
The type of lateral load resisting system to be used in astructure should be considered early in the planning stage
Lateral stability as well as architectural needs must be met
The three common lateral load resisting systems are:1. Braced Frames 2. Rigid Frames 3. Shear Walls
(Adapted from AISC 2002) 21
1 3
Rigid Horiz. Diaphragm
(Floor or Roof)
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5Braced Frames and Rigid Frames
This presentation focuses on braced frames (left) and rigid frames (right)
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6Steel Frame Connection Types
The Specification for Structural Steel Buildings (AISC 2005) defines two types ofconnections: Simple Connections (above left)
Moment Connections (above right)
Fully-Restrained and Partially-Restrained
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7 All connections have a certain amount of rigidity
Simple connections (A above) have some rigidity, but areassumed to be free to rotate
Partially-Restrained moment connections (B and C above)are designed to be semi-rigid
Fully-Restrained moment connections (D and E above) aredesigned to be fully rigid
Steel Frame Connection Types
(AISC)
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8Simple Connections
Designed as flexible connections
Connections are assumed to be free to rotate
Vertical shear forces are the primary forces transferred by the connection
Require a separate bracing system for lateral stability
The following few slides show some common simple framing connections
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9Common Simple Connections
Single Plate Connection (Shear Tab)
A plate is welded to the supporting member and bolted to the web of the
supported beam
Double Angle Connection
The in-plane pair of legs are attached to the web of the supported
beam and the out-of-plane pair of legs to the flange or web of the
supporting member
(Green, Sputo, and Veltri)
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Common Simple Connections
Shear End Plate Connection
A plate is welded perpendicular to the end of the supported web and
attached to the supporting member
Single Angle Connection
One leg is attached to the web of the supported beam and the other leg to the flange or web of the supporting
member
(Green, Sputo, and Veltri)
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Common Simple Connections
Seated ConnectionAn angle is mounted with one leg
vertical against the supporting column, and the other leg provides a
seat upon which the beam is mounted
A stabilizer connection is also provided at the top of the web
Tee ConnectionThe stem of a WT section is
connected to the supported member and the flange attached to the
supporting member
(Green, Sputo, and Veltri)
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Moment Connections
Designed as rigid connections which allow little or no rotation
Used in rigid frames Moment and vertical shear forces are transferred through the connection
Two types of moment connections are permitted:
Fully-Restrained Partially-Restrained
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Moment Connections
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Common FR Connections
Bolted Flange Plate ConnectionWelded Flange Plate Connection
Top and bottom flange-plates connect the flanges of the supported member to the supporting column
A single plate connection is used to transfer vertical shear forces
(Green, Sputo, and Veltri)
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Common FR Connections
Bolted Extended End-Plate Connection
A plate is welded to the flanges and web of the supported member and
bolted with high-strength bolts to the supporting column
Welded Flange Connection
Complete-joint-penetration groove welds directly connect the top and bottom flanges of the supported
member to the supporting column
A shear connection on the web is used to transfer vertical shear forces(Green, Sputo, and Veltri)
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Common PR Connections
PR Moment Connection Wind Only
A double angle simple connection transfers vertical shear forces while top and bottom flange plates resist moment forces produced by wind
Note that the size of the flange plate is relatively small in comparison to the
beam flange
Top and Bottom Angle with Shear End Plate Connection
Angles are bolted or welded to the top and bottom flanges of the supported member and to the
supporting column
A shear end plate on the web is used to transfer vertical shear forces
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Rigid Frames
Rigid frames, utilizing moment connections, are well suited for specific typesof buildings where diagonal bracing is not feasible or does not fit thearchitectural design
Rigid frames generally cost more than braced frames
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Braced Frames
Diagonal bracing creates stable triangular configurations within the steelbuilding frame
Braced frames are often the most economical method of resisting wind loads inmulti-story buildings
Some structures, like the one pictured above, are designed with a combinationbraced and rigid frame to take advantage of the benefits of both
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Structural steel frames requiretemporary bracing duringconstruction
Temporary bracing is placedbefore plumbing up thestructural frame
This gives the structuretemporary lateral stability
Temporary bracing is removedby the erector
Temporary Bracing
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In a braced frame, temporary bracing is removed after final bolt-up is completeand the permanent bracing system is in place
In a rigid frame, temporary bracing is removed after final bolt-up is complete
Temporary Bracing
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Concentric Braced Frames
Bracing is concentric when the center lines of the bracing members intersect
Common concentric braced frames used in buildings today include: X brace (above left) Two story Xs
X bracing is possibly the most common type of bracing
Bracing can allow a building to have access through the brace line dependingon configuration
Chevron (above right) Single diagonals
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X Bracing
The diagonal members of X bracing go into tension and compression similar toa truss
The multi-floor building frame elevation shown above has just one braced bay,but it may be necessary to brace many bays along a column line
With this in mind it is important to determine the locations of the braced bays ina structure early in a project
X-braced building elevation
X Bracing
Typical floor plan with X bracing
1st Floor
Floor
Floor
Roof
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X Bracing
Connections for X bracing are located at beam to column joints
Bracing connections may require relatively large gusset plates at the beam tocolumn joint
The restriction of space in these areas may have an impact on the mechanicaland plumbing systems as well as some architectural features
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Chevron Bracing
The members used in Chevron bracing are designed for both tension andcompression forces
Chevron bracing allows for doorways or corridors through the bracing lines in astructure
A multi-floor frame elevation using Chevron bracing is shown above
Chevron Bracing
Typical floor plan with Chevron bracing Elevation with several bracing configurations
Roof
1st Floor
Floor
Floor
Chevron V K
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Chevron bracing members use two types of connections
The floor level connection may use a gusset plate much like the connection onX braced frames
The bracing members are connected to the beam/girder at the top andconverge to a common point
If gusset plates are used, it is important to consider their size when laying-outmechanical and plumbing systems that pass through braced bays
Chevron Bracing
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Eccentrically Braced Frames
Eccentric bracing is commonly used in seismic regions and allows for doorwaysand corridors in the braced bays
The difference between Chevron bracing and eccentric bracing is the spacebetween the bracing members at the top gusset connection
In an eccentrically braced frame bracing members connect to separatepoints on the beam/girder
The beam/girder segment or link between the bracing members absorbsenergy from seismic activity through plastic deformation
Eccentric Brace
Link
Gusset
Eccentric brace with typical brace to beam connection (Adapted from AISC 2002)
Beam or Girder
Stiffeners
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Eccentrically Braced Frames
Eccentrically braced frames look similar to frames with Chevron bracing
A similar V shaped bracing configuration is used
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Eccentrically Braced Frames
Eccentric single diagonals may also be used to brace a frame
(EERC 1997)
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Combination Frames
As shown above (left) a braced frame deflects like a cantilever beam while amoment resisting frame deflects more or less consistently from top to bottom
By combining the two systems, reduced deflections can be realized
The combination frame is shown above right
Chevron braced
Moment resisting
Combination FrameBracing
Moment frame
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Combination Frames
The plot shows the moment resisting frame alone, the braced frame alone, andthe combined frame
The same wind load was used for each frame model
(AISC 1991)
O = Combined FramesX = Chevron or K Bracing = Moment Resisting
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ReferencesMaterial has been adopted from:
Earthquake Engineering Research Center, (EERC). (1997). W. G. GoddenStructural Engineering Slide Library. Godden J119. Available at: http://nisee.berkeley.edu/bertero/html/recent_developments_in_seismic_design_and_construction.html. Viewed August, 2004.
Green, P. S., Sputo, T., and Veltri, P. (n.d.). Connections Teaching Toolkit A Teaching Guide for Structural Steel Connections. American Institute of Steel Construction, Inc. Chicago, IL.
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